As reliance upon the internal combustion engine grows in our modern society, several driving factors have led to the demand for changes in the design and operation of such engines. As the available fuel supply slowly decreases, it is an objective of internal combustion engine design to provide for improved fuel consumption and economy while not adversely impacting the desired performance of such engines. Alternatively, increasing demands to satisfy environmental concerns associated with such engines have led to the need for providing complex systems to reduce emissions of noxious gases such as unburned hydrocarbons and nitrous oxides formed in the combustion process of internal combustion engines. These objectives in the design of internal combustion engines create conflicting design problems to a great extent as methods used to reduce emissions of noxious gases have tended to increase fuel consumption and vice versa.
In conventional spark ignition internal combustion engines, a gasoline type fuel is utilized in the combustion process. Conventionally, an air to fuel ratio which allows ignition of the mixture by use of a spark plug or the like must be provided. It is well known that the air/fuel mixture can only be ignited by a spark from a spark plug if the ratio of air to fuel is of the correct stoichiometric proportions. To achieve this, the volume of fuel and air introduced into a combustion chamber must be closely regulated in accordance with a particular engine load condition which will vary greatly upon operation of the engine. Thus, under all load conditions, the air to fuel ratio must be maintained within close tolerances and in accordance with the desired engine load which is normally accomplished by means of carburetors or fuel injectors with the volume of air/fuel varied using throttling techniques. The technology with respect to various liquid or gas fuels used in internal combustion engines has developed to the point where combustion efficiencies of the fuel are relatively high. Thus, the problems of inefficient or incomplete burning of the fuel within the air/fuel mixture in the combustion process are created to some degree by improper mixing of the air and fuel in the correct stoichiometric proportions which can be readily ignited by means of the spark.
To achieve higher efficiency using hydrocarbon fuels, the heat energy of the fuel should be released at the proper time in the engine cycle to harness the combustion efficiency of the fuel and facilitate more complete burning thereof. Theoretically, a lean fuel to air ratio increases thermal efficiency as the amount of fuel in the mixture decreases creating higher flame temperatures and thereby creating more work energy in the engine and allowing for more complete burning fuel. Conversely, a lean mixture also tends to increase the formation of nitrous oxides. Unfortunately, the lean fuel to air ratios desirable to achieve more complete burning and higher thermal efficiency in engine operation do not coincide with the air/fuel mixture requirements associated with spark ignition internal combustion engines.
It is also found that the effects of throttling conventionally utilized in internal combustion engines to achieve the desired output for a given load conditions create "pumping" losses in operation of the engine. The use of throttle plates in the conventional engine create vacuum conditions which are adverse to proper engine operation. For example, during part throttle, high vacuum conditions, a conventional spark ignition engine would require a great amount of ignition advance to allow enough time to complete combustion prior to exhausting the by-products of the combustion process. Ignition advance is necessary due to the slowing of the burning process under vacuum conditions wherein the fuel mixture will require more time to burn. The necessity of operating the engine in this manner causes pressure build up in the cylinder head before top dead center of piston travel within the engine, thereby causing some of the thermal energy created in the combustion process to be wasted as it is essentially being used against the direction of crank shaft rotation. Additionally, under part throttle conditions in conventional engines, the combustion process cannot be optimized, and burning of the fuel occurs at relatively high temperatures. This creates disassociation of the combustion products while slowing combustion thereby resulting in unwanted emissions and incomplete burning of the fuel. Similarly, during full power higher RPM operation of a conventional spark ignition engine, again a great amount of ignition advance is required as less time is available for fuel burn to take place in the combustion chamber. Similar problems as discussed relative to part throttle engine operation are thus encountered. It is therefore seen that at almost any load condition, ignition advance requirements result in undesirable effects in not effectively utilizing the expansive energy created by combustion which can be converted to usable rotary torque.
Other deficiencies in the conventional spark ignited internal combustion engine are also apparent. A conventional engine has a calculated theoretical compression ratio of about 9 to 1. Under part throttle conditions, the effective compression ratio is normally far below the desired theoretical compression ratio and is commonly found to be 3 to 1 or less. This condition exists again due to the use of throttle plates designed to produce lower power under part throttle conditions. Additionally, compression ratios are limited in conventional engines to around 9 or 10 to 1 by the phenomena of preignition which occurs in the both part or full throttle modes. The phenomena of preignition occurs when pressure and temperature become high enough to burn fuels spontaneously in different areas of the combustion chamber after spark ignition has occurred, but before the normal burning of the fuel mixture has reached all areas within the combustion chamber. This limitation on the achievable compression ratio within the engine in turn places upper limits on achievable fuel economy and power, and the ability to optimize use of the heat energy released during combustion by not allowing higher pressures within the cylinder.
Various design modifications have attempted to account for some of the deficiencies found in the conventional engine. For example, the use of precombustion chambers have been utilized to provide better combustion characteristics within the main combustion chamber. In a standard design of this type, a rich fuel to air mixture is admitted into the pre-combustion chamber and ignited externally of the conventional engine cylinder. The ignited fuel mixture is then injected into the main combustion chamber of the engine cylinder which has a fuel mixture which is a more lean mixture than could be ignited by spark. The design allows more efficient burning of the mixture in the main combustion chamber but has added complexity to the design as well as significantly to the cost of the engine. The small precombustion chambers have also been found to present problems with respect to cleaning and maintenance to ensure proper functioning. Some examples of the use of pre-combustion chambers are found in U.S. Pat. Nos. 3,994,267 and 4,784,098. These patents also utilize a stratified charge combustion process for attempting to optimize combustion characteristics over the range of load conditions encountered in operation of the engine. Other techniques attempt to achieve better mixing of the air and fuel injected into the combustion chamber or enrichment techniques as found in U.S. Pat. Nos. 4,498,434, 4,788,742 and 4,829,958. Again, the complexities and cost of these systems inhibit their effective use.
Some of the deficiencies found in the design of internal combustion engines have been addressed in part by diesel engine design. Specifically, diesel engine design eliminates the use of throttling plates, and the need for throttling, which directly results in higher thermal efficiency and fuel economy. The diesel engine supplies the amount of fuel needed for the particular load condition which the engine is under, and combustion of the fuel occurs due to the temperature/pressure in the cylinder.
Although the diesel engine seemingly provides better fuel economy, the fuel is not burned fast enough in many situations, resulting in high emissions of noxious materials which is at odds with environmental concerns. The diesel engine is basically a very "dirty" in its operation, as it produces a considerable amount of particulate matter, other noxious aeromatics and nitrous oxides in its emissions. The diesel engine is also very efficient because it does not use throttle plates, and can burn fuel very lean and has very high compression ratios. The diesel engine produces more power per unit of fuel burned, but is limited to lower rpms compared to a gasoline engine. Thus, even though the torque produced by the engine is high, it cannot turn high rpms to produce high horse power like a gasoline engine. When the diesel engine is used with automobiles, some of the advantages are also negated. In order to achieve the output desired from the engine, diesel engines require high working pressures which in turn require heavy duty materials to provide sufficient strength to accommodate such pressures. Such considerations add significantly to the weight of the diesel engine as well as the cost of producing the engine resulting in a design which reduces the fuel economy advantages obtainable in theory. Heavy duty materials are also necessary to due a form of preemission caused by diesel fuel not burning rapidly in the engine. This preignition phenomenon makes it necessary for make all engine parts heavier then otherwise necessary in order to avoid breaking thereof. The preignition caused by the fuel not burning rapidly enough in the diesel engine also creates a great amount of noise and vibration in the operation of the engine in automobiles, which is an undesirable aspect for the consumer. The high speeds associated with automotive use of diesel engines has led to the development of various specialized designs to improve the combustion characteristics of high speed diesel engines.
In particular, diesel engine design has resorted to the use of a swirl chamber associated with a main combustion chamber to allow a more efficient mixing and burning of the fuel in high speed diesel engines. In U.S. Pat. No. 4,662,330, a swirl chamber type diesel engine includes a modified "clover leaf" type recess in the piston crown which promotes desirable flame dispersion and good mixing of the flame with the air in the main combustion chamber. This in turn allows the development of the desired compression ratio during the compression phase of the engine cycle. Similarly, in U.S. Pat. No. 4,122,804, a diesel engine includes a combustion chamber arrangement including a precombustion chamber having a discharge passage which is in alignment with a pocket formed under the valve head adjacent the piston and at the piston top dead center position. Air discharge into the prechamber during each compression stroke generates a swirl which is directed to the pocket formed in the construction so as to provide a temporarily locally rich mixture wherein combustion occurs under conditions relatively unfavorable to nitrous oxide production. Although the use of swirl chambers and special constructions such as shown in these prior patents allow comparatively high engine speeds, fuel economy is adversely impacted and efficiency reduced.
It therefore would seem to be desirable to provide an internal combustion engine which attempts to achieve advantages which are theoretically obtainable in diesel engines for use with high speed engines for automotive use, while avoiding the disadvantages which have been found in attempting to design diesel engines for such requirements. It is additionally thought to be desirable to provide an internal combustion engine which will perform with higher efficiency while correspondingly reducing unburned hydrocarbons and nitrous emissions. In order to accomplish such characteristics, the required amount of ignition advance should be reduced to allow a greater proportion of the combustion process to occur at or after top dead center of piston travel within the engine to achieve the greatest amount of torque generation from the engine.
One attempt at providing an engine with diesel-like fuel economy and low emissions of carbon dioxide, without the deficiencies of noise, exhaust fumes and other problems associated with diesel engines has been proposed recently. In an experimental 1.7-liter 4-cylinder engine developed by Volkswagen, a direct injection fuel system developed by Stanadyne Automotive Corp. is utilized. The engine is claimed to resemble a diesel engine in many respects, but in a spark ignition type engine. An ultra-lean air/fuel mixture is used along with a high compression ratio. In the construction, combustion chambers are recessed in the piston crowns, and a pencil-slim nozzle allows fuel injectors fitted in the cylinder head to be positioned at the edge of each combustion chamber to introduce liquid gasoline tangentially to the cylinder wall. A two-stage injection process achieves a peak fuel pressure in the combustion chamber after which ignition occurs to prevent preignition effects. A special spark plug configuration including a long central electrode and a trio of ground electrodes help to provide proper combustion characteristics. In this engine, it is stated that the output is controlled by the amount of fuel sprayed, and not by how much air is emitted through a throttle plate so as to simplify the engines induction plumbing and allowing reductions in pumping losses. Problems are thought to be present in this system such as inability to adjust the system for optimizing fuel economy and desired emission characteristics.